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Bartonellosis: An emerging and potentially hidden epidemic?

Bartonella species, their animal hosts, potential vectors, and sequelae of infection are being identified at a snowballing rate. A new diagnostic test may help DVMs and MDs come together to better understand these infections in their patients.

Veterinarians and physicians should talk to each other more frequently than we have in the past. Of all known organisms, 61% are zoonotic,1,2 and of the emerging pathogens, the vast majority are zoonotic organisms. My research at the Intracellular Pathogens Research Laboratory at the North Carolina State University College of Veterinary Medicine focuses on vector-transmitted organisms, of which 22% are zoonotic.1,3 In recent years, the genus Bartonella has been the major focus of our vector-borne research efforts.

BARTONELLA SPECIES COMPLEXITIES

The organism that causes cat scratch disease in people was identified as a Bartonella species in 1992. As researchers have continued to study these bacteria, we've learned that the intraerythrocytic component of the infection with a Bartonella species has been somewhat overemphasized. My research laboratory and others have demonstrated that Bartonella species are endotheliotropic bacteria that use a specialized invasion process to enter endothelial cells and can move about the body by infecting macrophages, with localization in a variety of tissues.3

Knowing more about Agrobacterium species and the pathogenic mechanisms it uses to induce tumors in plants will probably benefit physicians and veterinarians in better understanding what Bartonella species are doing to promote vasculoproliferative disorders in our patients.4

Researchers have learned that Bartonella species are the first bacteria identified to have an ability to invade CD34+ progenitor cells in bone marrow.5 This may be why we find Bartonella organisms in cats in only a low percentage (3%) of their erythrocytes.

The genus Bartonella is also unusual because it appears that no other infectious agent is transmitted by more vectors. We now know that sand flies, human body lice, cat fleas, rodent fleas, and probably many other flea species are capable of transmitting certain Bartonella species. And cattle, deer, elk, and sheep all have their own Bartonella species that appear to be transmitted by biting flies or keds (wingless flies).6

Our laboratory wondered whether ticks could transmit Bartonella species. Currently, the answers appear to be yes and maybe.7 Good case-based evidence exists in the human and veterinary medical literature that suggests ticks may transmit Bartonella species. And in many laboratories around the world, PCR testing reveals Bartonella species DNA in ticks, particularly in Ixodes species ticks. Researchers in France have nearly demonstrated Ixodes ricinus transmission of Bartonella henselae.8 Interestingly, Bartonella henselae is prevalent in I. ricinus ticks in southern Germany and in France.9 However, although tick transmission of Bartonella species is possible, it hasn't yet been definitively proven for ticks in North America.

IS A HIDDEN EPIDEMIC POSSIBLE?

In my opinion, a hidden epidemic is possible if several conditions are met.

First, you start with an unknown bacterial genus, which was the situation for Bartonella in North America and much of the world until the human immunodeficiency virus (HIV) epidemic of the late 1980s and 1990s.

Second, the organism must behave as a stealth pathogen that is capable of hiding in the host, as is true for Bartonella species.10

Third, a large number of diverse animal reservoir hosts for the organism are maintained in nature—and that too is the case for the genus Bartonella. Several mammals—grey squirrels, flying squirrels, groundhogs—have their own genetically distinct Bartonella species that have coevolved in those animals and have a high prevalence in their respective hosts in nature.

Finally, the organism creatively facilitates its transmission, not only through vectors, but by direct transmission through bites and scratches, which has been reasonably well-established for dogs and cats and for rabbits in Europe.11